two different levels in the sky and sometimes in
conjunction with altocumulus, which may also exist as
two different layers in the sky.
Nimbostratus (NS). Nimbostratus appears as a
low, amorphous, and rainy layer of clouds of a dark
gray color. They are usually nearly uniform and feebly
precipitation occurs, it is in the form of continuous rain
or snow. However, nimbostratus may occur without
rain or snow reaching the ground. In cases in which the
precipitation does not reach the ground, the base of the
cloud is usually diffuse and looks wet. In most cases,
nimbostratus evolve from altostratus layers, which
grow thicker and whose bases become lower until they
become a layer of nimbostratus.
Low clouds. Low clouds are described as follows:
Stratocumulus (SC). Stratocumulus appear as a
layer (or patches) of clouds composed of globular
masses or rolls. The smallest of the regularly arranged
elements is fairly large. They are soft and gray with
Stratus (ST). Stratus appears as a low, uniform
layer of clouds, resembling fog, but not resting on the
ground. When a layer of stratus is broken up into
irregular shreds, it is designated as stratus fractus. A
veil of stratus gives the sky a characteristically hazy
appearance. Usually, drizzle is the only precipitation
associated with stratus. When there is no precipitation,
the stratus cloud form appears drier than other similar
forms, and it shows some contrasts and some lighter
Cumulus (CU). Cumulus is dense clouds with
vertical development. Their upper surfaces are dome
shaped and exhibit rounded protuberances, while their
bases are nearly flat. Cumulus fractus or fractocumulus
resemble ragged cumulus in which the different parts
show constant change.
Cumulonimbus (CB). Cumulonimbi are heavy
masses of cumulus-type clouds with great vertical
development whose cumuliform summits resemble
mountains or towers. Tops may extend higher than
60,000 feet. Their upper parts are composed of ice
crystals and have a fibrous texture; often they spread
out in the shape of an anvil.
Cumulonimbi are the familiar thunderclouds, and
their precipitation is of a violent, intermittent, showery
cumulonimbus. On occasion, cumulonimbus clouds
Examples are (1) mamma or hanging
pouch-like protuberances on the under surface of the
cloud; (2) tuba (commonly called the funnel cloud),
cone/pendant from the cloud base; and (3) virga, wisps
or streaks of water or ice particles falling out of a cloud
but evaporating before reaching Earths surface as
The Aerographers Mate must learn to recognize
the various cloud types and associated precipitation as
seen from Earths surface. Figure 5-1 shows the various
types of clouds in a tier with each cloud type at its
average height. Although one never sees all cloud types
at once, quite frequently two or three layers of clouds of
different types may be present simultaneously.
Species.The following species of clouds are
referred to frequently; others may be found in the
International Cloud Atlas or in the newer publication,
Cloud Types for Observers.
Castellanus. Clouds which present, in at least some
portion of their upper part, cumuliform protuberances
in the form of turrets. The turrets, which are generally
taller than they are wide, are connected to a common
base. The term applies mainly to cirrocumulus,
Stratiformis. Clouds which are spread out in an
extensive horizontal sheet or layer. The term applies to
Lenticularis. Clouds having the shape of lenses or
almonds, often elongated and having well-defined
outlines. The term applies mainly to cirrocumulus,
altocumulus, and stratocumulus.
Fractus. Clouds in the form of irregular shreds,
which have a clearly ragged appearance. The term
applies only to stratus and cumulus.
Humilis. Cumulus clouds of only a slight vertical
extent; they generally appear flattened.
Congestus. Cumulus clouds which are markedly
sprouting and are often of great vertical extent. Their
bulging upper part frequently resembles cauliflower.
Varieties and Supplementary Features.Cloud
varieties are established mainly on the basis of the
clouds transparency or its arrangement in the sky. A
detailed description of the nine varieties can be found in
the International Cloud Atlas.